1. Field of the Invention
The present invention relates to generally to optical sensors using waveguides and more specifically to optical sensors using a plurality of waveguides.
2. Description of the Background Art
The basic technique for fluorescence excitation and detection in sensors measuring surface reactions is based on evanescent field excitation and either evanescent field or far field detection. ((See Golden et al. An Evanescent Wave Fiber Optic Biosensor: Challenges for Real World Sensing, reprinted from Chemical, Biochemical and Environmental Fiber Sensors IV in 1796 SPIE Proceedings Series, pp. 2-8 (Meeting 8-9 September 1992, in Boston, Mass.; published April 1993), Kroneis et al. U.S. Pat. No. 4,703,182, Oct. 27, 1987, Ligler et al. Array Biosensor for Multi-Analyte Sensing, SPIE Proceedings Series, in press (1998).) For single analyte detection, a principal method is a fiber-based optic biosensor (see Anderson et al., IEEE Eng. Med. Biol., 13, 358-363 (1994)), which use both evanescent field excitation and detection. Although highly successful, it has been found that the sensitivity of this method is limited, in part, by the unavoidable background of excitation light. Technologies for multi-analyte, multi-sample detection such as the array biosensor which uses evanescent field excitation from a planar waveguide and point by point imaging in the far field with a CCD are also susceptible to background scatter from the excitation light (see Feldstein et al., J. Biomed. Microdevices, 1, 139-153 (1999)). In addition, CCD imaging for biosensor applications is limited by the cost of a high quality imaging element and the requisite signal collection and processing power.
Evanescent field excitation of bound fluorophores has the benefit of localizing the excitation field at the sensor surface. This allows, in principle, detection of bound species even in the presence of unbound fluorophores in the bulk solution. However, in practice, bulk excitation and scatter are significant, and measurements are typically made in the absence of a bulk solution. Moreover, the evanescent field generated by total internal reflection at an interface is weak (in the absence of an enhancement mechanism such as surface plasmon polariton excitation) as compared to that which can be achieved by direct illumination from a source of equivalent power.
The issue of the lateral confinement of the optical excitation and signal is also of significant import to these optical methods, especially with respect to the array system. Specifically, when using a single planar waveguide to distribute excitation to multiple recognition elements, there can be cross talk between the elements since there is an absence of lateral optical confinement between these elements. Moreover, uniformity of excitation is difficult to achieve in a laterally multi-mode waveguide. Hence, recognition elements may be exposed to differing excitation intensities, thus making a single uniform system calibration impossible.
Finally, spatially distributed recognition elements (which yield spatially distributed fluorescence) have consequences for optical detection methods and signal to noise optimization. For example, in biosensors utilizing CCD imaging, the detection sensitivity is limited by the large number of pixels, each of which have inherent noise. Based on considerations such as this, it would be preferable to spatially integrate all the fluorescent emission to a single detection element.